The present invention is directed to air traps, and is particularly concerned with an arrangement for trapping and removing air or other gas bubbles from a liquid chromatography system.
In liquid chromatography a liquid sample is passed by a flowing stream of liquid solvent (the mobile phase) through a column packed with particulate matter (stationary phase). While passing through the column the various components in the sample separate from one another by adsorbing and desorbing from the stationary phase at different rates such that these individual components elute from the column at different times. The separated components then flow through a detector which responds to each component both qualitatively and quantitatively thereby providing information to the user about the separation achieved by the chromatography column.
The particulate matter in the chromatography column is generally referred to as the chromatography media and the resolution of a separation of the stream into individual components by the chromatography media is a primary measure determinant of the economic value of the chromatography.
Accordingly it is necessary to protect the chromatography media from conditions which will ruin the media including the presence of air and the presence of microbial contamination.
The presence of air (or other gas—in the following, in the interests of brevity, “air” and “gas” are used interchangeably to mean air and/or any other gas) in a chromatography column fills the pores of the chromatography media and blocks the liquid sample from getting to the active sites of the media. Additionally the presence of air disrupts the flow of the liquid sample which makes it impossible to achieve an even flow of the liquid sample through the column Additionally, dissolved air in the liquid may form bubbles under conditions of reduced pressure which are often present after the outlet of a chromatography column and these bubbles can interfere with the performance of detectors used to register the presence or absence of components of interest in the liquid leaving the column.
Microbial or bacterial contamination is a particular problem with low pressure chromatography because the liquid phase is aqueous and the gel media a carbohydrate. The presence of air can allow bacteria to grow which interferes with the ability of the media to produce a clean, pure product.
Accordingly it is highly desirable to prevent air from entering the column and to provide a air trap that not only excludes air from entering the column but also is of a sanitary design which inhibits the growth of bacteria within the chromatography system and, in particular the chromatography column Air traps are a known technique in liquid chromatography for protecting the chromatography column and more specifically the chromatography media. A typical air trap comprises a reservoir located at the down stream end of a system pump for delivering a liquid to a chromatography column The reservoir supplies the liquid to the chromatography column Any air whether from air entrapped in the liquid, or resulting from a slow leak or from a dry line condition would be trapped in the upper portion of the reservoir above the level of the liquid therein.
An example of such an air trap is known from U.S. Pat. No. 5,112,492. This teaches that the air trap may be positioned in a liquid chromatography system at the down stream end of the system pump and upstream of the chromatography column The air trap functions as a reservoir for receiving the entire volume output of the system pump and is provided with an inlet and an outlet (each provided with a valve) in the base of the reservoir and a top exhaust valve. The inlet and outlet valves are normally opened during operation while the exhaust valve is normally closed. A bypass valve may be located between the inlet and outlet valves to the air trap so that the air trap can be bypassed for certain operations such as sample loading of the liquid column The reservoir includes level detection devices at spaced elevations in the reservoir for the purpose of maintaining the liquid level above the outlet.
The operation of the air trap is controlled by a controller which receives inputs from the level sensors and sets the valves on the automated air trap according to the controller design. During normal operation, the air trap controls the level of the liquid between the lower and upper sensing levels and will shut down the operation in the event the liquid level can not be maintained between these two elevations.
During normal operation, the air trap will have inlet and outlet valves open. If the liquid level falls below the lower elevation, the system infers that air is being detected at the lower elevation, the outlet valve will close and the exhaust valve will open until the liquid level rises and it is detected at the upper elevation. If upper level detection of liquid does not occur within a preset time period, which will be dependent on the refill flow rate and on the air trap volume, then the system will shut down because a dry inlet line condition is inferred and the condition must be corrected before continuing operation.
In order to avoid bubbles in the incoming liquid being drawn into the outlet of the reservoir it is necessary to provide a distance between the inlet and outlets which is sufficiently long enough to allow incoming bubbles to rise to the top of the reservoir before the volume of liquid that they were present in leaves the reservoir. This requires a large reservoir volume which leads to an undesirable delay in the time it takes the sample being supplied to the chromatography column to reach the column.